Biocompatible nanoparticles (NPs), including quantum dots (QDs), exploit the unique size-dependent properties of their constituent materials and can serve various important roles in a biological context. For example, such nanoparticle materials have attracted significant interest in their use for cellular labeling and in vivo imaging studies. Various uses of nanoparticles are described in, e.g., Algar et al, Bioconjugate Chem. 2011, 22, 825-858 (2011); U.S. Pat. No. 7,858,386; and U.S. Patent Application Publication Nos. 2008/0087843, 2011/0089241, and 2011/0097797, each of which is incorporated herein by reference.
For use of semiconductor quantum dots (QDs) and gold nanoparticles (AuNPs) in biological applications, good pH and ionic stability as well as small hydrodynamic volume are desirable. These properties of NPs can depends on which “ligand” coats the surface of the NPs to provide aqueous stability to the NPs and facilitate their integration within biological systems. Various families of ligand molecules have been utilized for these purposes, with each usually providing its own set of functional benefits and liabilities. However, a need remains for compact multifunctional ligands for nanoparticles that provide a desired combination of features.